Stars as Laboratories for Fundamental Physics - MPP Theory Group

Neutrinos: The Bottom Line 575
is dominated by the plasmon decay process γ → νν for a large range
of stellar conditions (Appendix C). This process occurs mostly by the
vector-current coupling to electrons of the medium. Because the weak
mixing angle has the special value sin 2 Θ W = 0.23 ≈ 1 the vectorcurrent
coupling of ν µ and ν τ to electrons nearly vanishes (Appendix B)
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and so γ → ν µ ν µ and ν τ ν τ plays near to no role anyway—in this sense
stars are “blind” to the issue of heavy ν µ or ν τ masses.
However, assuming that the heavy neutrinos are Dirac particles one
can limit their mass by a SN argument: The energy loss by right-handed
states produced in spin-flip collisions must not be too large, yielding a
limit of m < ν ∼ 30 keV (Sect. 13.8.1). In addition, these neutrinos have
very high energies, typical of SN core temperatures, and so their decays
might produce high-energy daughter ν e ’s (100−200 MeV) which were
not observed from SN 1987A (Sect. 13.8.1).
Even Majorana neutrinos would not be harmless for SN physics if
they would mix sufficiently strongly with ν e . In this case they would
effectively participate in β equilibrium (Sect. 9.5) so that the spinflip
scattering of, say, ν τ would effectively lead to ν τ ’s and thus to
deleptonization without the need of transporting lepton number to the
stellar surface.
In addition, even though heavy Majorana neutrinos would be emitted
from the neutrino sphere so that their decays would not produce
high-energy daughter products, the decays could still add to the detectable
signal. It is not obvious from the existing literature which
(if any) range of masses and decay times is ruled out or ruled in by the
SN 1987A observations (Sect. 13.2.2).
If the fast decays were due to some sort of majoron model, large
“secret” neutrino-neutrino interactions are conceivable, possibly in conjunction
with a small vacuum expectation value of a new Higgs field so
that the symmetry may be restored in a SN core. While a substantial
body of literature exists on this sort of scenario (Sect. 15.7.2) I believe
that in this context the story of SN physics would have to be rewritten
more systematically than has been done so far. However, there appears
to be little doubt that for neutrino-majoron Yukawa couplings in excess
of about 10 −5 one would expect dramatic modifications of the transport
of energy and lepton number.
In summary, “heavy” neutrinos with fast invisible decays are a way
to circumvent the cosmological mass limit, and may indeed be desirable
in certain scenarios of cosmic structure formation. Depending on their
detailed properties they could have a substantial impact on SN physics
and the signal observable in a detector. It is difficult, however, to